1. Field of the Invention
This invention relates to drive method and drive apparatus for gradation display of each pixel in a capacitive display device such as electroluminescence (EL) display device.
2. Description of the Prior Art
For instance, a double insulation type (or triple insulation structure) thin film EL element is composed as follows.
As shown in FIG. 1, strips of transparent electrodes 2 made of IN.sub.2 O.sub.3 are disposed parallel on a glass substrate 1, and an inductive substance layer 3a of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 or the like, an EL layer made of ZnS doped with activator such as Mn, and a similar inductive substance layer 3b of Y.sub.2 O.sub.3, Si.sub.2 N.sub.4, TiO.sub.2, Al.sub.2 O.sub.3 or the like are sequentially laminated thereon in a film thickness of 500 to 10000 .ANG. by thin film technique such as evaporation or sputtering method. Thereby, a three-layer structure is composed strips of back electrodes 5 made of Al are then disposed parallel thereon in a direction orthogonal to the transparent electrodes 2.
Such a thin film EL element has the EL substance 4 sandwiched between the inductive substances 3a, 3b placed between its electrodes. It may be regarded as a capacitive element from the viewpoint of equivalent circuit. Further, as is clear from the voltage-brightness curve shown in FIG. 2, this thin film EL element is driven by applying a relatively high voltage of about 200 V. This thin film EL element emits light at a high brightness by an AC electric field, and has a long life.
The basic display drive of the thin film EL display apparatus using such a thin film EL element as a display panel is achieved by applying a writing voltage sequentially to the scanning side electrodes, along with a modulation voltage corresponding to the display data to determine emission and non-emission applied to the data side electrodes. This is achieved by using one of the transparent electrodes 2 and back electrodes 5 of the thin film EL element as scanning side electrodes, and the other ones as data side electrodes. By this display drive, the superimposing effect or canceling effect of the writing voltage and modulating voltage occurs at the intersecting pixel area of the scanning side electrode and data side electrode of the EL layer, and a voltage over the emission start voltage or a voltage under the emission start voltage is applied. Thereby, each pixel is set in emission state or non-emission state, so that a desired display is obtained.
Conventionally, in such thin film EL display apparatus, as a method of driving a gradation display for varying the brightness of each pixel in a plurality stages, the voltage modulation method of variably setting the modulation voltage to be applied to the data side electrode depending upon gradation display data (brightness data), has been known.
FIG. 3 is a circuit diagram showing one output port of the data side driver circuit of a thin film EL display apparatus intended to display in gradation by driving according to the voltage modulation method mentioned above, and FIG. 4 is a timing chart showing the operation of the same circuit. In FIG. 3, an input terminal 6 is a terminal for receiving a ramp waveform voltage V.sub.RA as shown in FIG. 4 (1). This input terminal 6 is connected to one of the terminals of a capacitor 8 through a switch 7, while the other terminal of the capacitor 8 is grounded. The connecting point of the switch 7 and capacitor 7 is connected to each gate of an N-channel MOS transistor 9 and a P-channel MOS transistor 10.
The drain of the N-channel MOS transistor 9 is connected to a power supply 11 for feeding a voltage HVCC corresponding to the maximum of modulation voltage applied to the data side electrode, the source of the transistor 9 is connected to the source of the P-channel MOS transistor 10, and the drain of the transistor 10 is grounded. The connecting point of the source of N-channel MOS transistor 9 and the source of the P-channel MOS transistor 10 is connected to an output terminal 12.
In the thus composed data side drive circuit, when a ramp waveform voltage V.sub.RA shown in FIG. 4 (1) begins to be applied to the input terminal 6, the switch 7 is turned on at the same time. The ON duration of the switch 7 is set according to the gradation display data mentioned above. While the switch 7 is being turned on, an electric current flows into the capacitor 8, and the capacitor 8 is charged to a voltage depending on the ON duration of the switch 7 (from time t.sub.0 to t.sub.1 in FIG. 4), that is, the gradation display data. This charging voltage is applied to the gate, the N-channel MOS transistor 9 is turned on, the P-channel MOS transistor 10 is turned off, and the output terminal 12 delivers an output voltage as shown in FIG. 4 (3), corresponding to the charging voltage of the capacitor 8, or the modulation voltage Vm corresponding to the gradation display data. This modulation voltage Vm is applied to the data side electrode, and by the difference in this modulation voltage Vm, the brightness of the corresponding pixel varies, and gradation display is effected.
In this drive method, however, the N-channel MOS transistor 9 is not always turned on in saturated state, but the ON resistance varies with the voltage applied to the gate, or the gradation display data, and hence it is a considerably high value. On the other hand, the thin film EL element is a large-sized capacitive display element, and the quantity of current flowing per channel (a circuit for one pixel) in the drive circuit is large, and the heat loss at the N-channel MOS transistor 9 becomes very significant. It is hence difficult to integrate the transistors when composing a drive circuit in an integrated circuit. Further, in the manufacturing process of the integrated circuit, since the P-channel MOS transistor and N-channel MOS transistor of high voltage resistance must be assembled, the manufacturing cost of integrated circuit is very high, which has made it hard to realize practically.
Further, in such thin film EL display apparatus, as a drive method of gradation display for varying the brightness of pixels in plural stages, the pulse width modulation method for varying the pulse width of the modulation voltage applied to the data side electrode depending on the gradation display data (brightness data) and controlling the time-wise integrated value of the effective voltage applied to the pixels is considered.
In this drive method, however, the gradation brightness is not stable as mentioned below, and many stages of gradation cannot be set.
FIG. 5 (1), (2), (3) are an applied voltage waveform to pixels in one presupposed pulse width modulation method, a waveform of power supply current at this time, and a waveform of current flowing in the emission layer of pixels, displayed to explain the causes of such problems.
As shown in FIG. 5 (1), the effective voltage V.sub.A applied to a pixel is obtained as a superimposed value of the modulation voltage V.sub.M applied to the data side electrode. Further, a writing voltage V.sub.W is applied to the scanning side electrode in the reverse polarity of the modulation voltage V.sub.M and in a magnitude corresponding to the emission threshold voltage Vth. When the effective voltage V.sub.A of such a rectangular wave is applied to a pixel, the waveform of the power supply current that becomes as shown in FIG. 5 (2).
That is, while the effective voltage V.sub.A does not achieve the emission threshold voltage Vth, a nearly constant current not contributing to the emission flowing in the capacitive portion of pixel flows. Further, when the effective voltage V.sub.A exceeds the emission threshold voltage Vth, the current portion flowing in the emission layer of pixel that is, the current portion contributing to emission is added in addition to the current portion flowing in the capacitive component of the pixel, and the current flowing in the emission layer becomes that as shown in FIG. 5 (3). The emission brightness of pixels becomes larger in proportion to the current quantity of the current flowing in the emission layer.
Here, when the pulse width of the modulation voltage V.sub.M is limited as indicated by broken line in FIG. 5 (1), the current flowing in the emission layer is shut off at the fall point of the modulation voltage V.sub.M. That is, by limiting the pulse width of the modulation voltage V.sub.M, the quantity of current flowing in the emission layer of the pixel is controlled. Thus, a brightness corresponding to the pulse width of modulation voltage V.sub.M is obtained.
As mentioned above, however, when the effective voltage V.sub.A applied to pixels is a rectangular wave, that is, when the modulation voltage V.sub.M is a rectangular wave, the current flowing in the emission layer becomes a peak current, and its passing time is short (as indicated by t1 in FIG. 5 (1)). Thus, the pulse width of the modulation voltage V.sub.M cannot be set in multiple stages. This means that it is impossible to control the brightness in mutilple stages. Besides, at the brightness of each stage, since the current flowing in each emission layer becomes large, only a slight error in the pulse width of the modulation voltage V.sub.M may result in a large change in the brightness. This makes it difficult to stabilize the gradation of brightness.
In this drive method, for example, when transparent electrodes of high line resistance are used as data side electrode, the modulation voltage applied to the data side electrodes is affected by the line resistance, and a brightness difference occurs among pixels as described below.
FIG. 6 is a connection diagram of display panel 13 of a thin film EL display apparatus and part of its drive circuit, presented to explain the cause of such brightness difference. In FIG. 6, the data side electrodes 14a, 14b are connected with output ports 15a, 15b of the data side drive circuit for applying modulation voltage V.sub.M to these data side electrodes 14a, 14b, at the electrode end parts drawn out to the upper side in the drawing. On the other hand, a plurality of scanning side electrodes 16a, 16b, 16c, 16d are disposed mutually parallel in a direction orthogonal to the data side electrodes 14a, 14b. These scanning side electrodes 16a to 16d are respectively connected with output ports 17a, 17b, 17c, 17d of scanning side drive circuit for applying writing voltage -V.sub.W to them, at the electrode end parts drawn out to the left side in FIG. 6. In FIG. 6, meanwhile, intermediate line resistances of the data side electrodes 14a, 14b are indicated by resistance R.
In the thus composed thin film EL display apparatus, a writing voltage -V.sub.W, for example, corresponding to an emission threshold voltage Vth, is applied to the scanning side electrode 17a in order to emit two pixels 18A, 18D positioned on the data side electrode of an equal brightness, and the same writing voltage -V.sub.W is applied to the scanning side electrode 17d. Further, let us suppose in the example, that voltages of the same waveform are applied as modulation voltage V.sub.M applied from the output port 15a of the data side drive circuit to the data side electrode 14a.
At the pixel or picture element 18A at a position near the output port 15a, since the line length of the data side electrode 14a from the output port 15a to the pixel 18A is short, the effect of the line resistance is almost negligible. Therefore a voltage of nearly the same waveform as the modulation voltage V.sub.M, delivered from the output port 15A as shown in FIG. 7 (1), is applied to the pixel 18A. At this time, when a writing voltage -V.sub.M in the waveform as shown in FIG. 7 (2) is applied from the output port 17a of the scanning side drive circuit to the scanning side electrode 16a, an effective voltage in the waveform as shown in FIG. 7 (3) is applied to the pixel 18A.
By contrast, at the pixel 18D remote from the output port 15a, the line length of the data side electrode 14a from the output port 15a to the pixel 18D is long, the line resistance R in this length is large, and the modulation voltage V.sub.M is greatly affected by the line resistance R. Therefore, as shown in FIG. 8 (1), a voltage in an integral waveform, as if passing the modulation voltage V.sub.M delivered from the output port 15a into an integrating circuit, is applied to the pixel 18D. At this time, when a writing voltage -V.sub.W in the waveform as shown in FIG. 8 (2) is applied from the output port 17d of the scanning side drive circuit to the scanning side electrode 16d, an effective voltage in the form as shown in FIG. 8 (3) is applied to the pixel 18D.
Of the effective voltages applied to the pixels 18A, 18D, the voltage substantially contributing to the emission is the portion over the emission threshold voltage. Therefore, when the waveform in FIG. 7 (3) and the waveform in FIG. 8 (3) are compared with respect to the portion over the emission threshold voltage Vth, the waveform in FIG. 8 (3) is wider in the area by the shaded area portion. This area difference becomes a direct difference in the brightness, and between the pixels 18A, 18D, although it was intended to emit in the same brightness, the pixel 18D is actually brighter than the pixel 18A.
That is, if modulation voltage V.sub.M of an identical waveform is applied, the pixel closer to the output port is darker. Thus, the remoter one is brighter, and the brightness fluctuates, and when the brightness difference occurs between pixels adjacent vertically which should be identical in gradation, the display quality deteriorates.